A. Field
This invention relates to a method and corresponding apparatus for recognizing foreign material, in particular adhesive objects, glue or joints, on or in bank notes.
B. Related Art
In machine processing of bank notes in commercial or central banks, bank notes are checked for, among other things, their fitness for further use in payment transactions. An important criterion for decision is the presence of undesirable foreign objects on or in the notes. Such undesirable objects are generally objects that are not part of a finished bank note and are applied to or incorporated in the note only during circulation. They are frequently adhesive tape or similar adhesive objects that are stuck to the note in particular for purposes of repair. Occasionally it can happen that bank notes are provided with paper clips or staples for similar or other purposes. Such notes are generally regarded as unfit for further use and must be accordingly recognized, rejected and possibly destroyed during machine processing.
For recognizing undesirable objects on bank notes in bank note processing machines according to the prior art, optical methods are used, for instance, by which the light reflected by a note to be checked or transmitted through the note is detected. The detected light is used to derive statements about the presence of foreign material on the note. However, this method delivers insufficiently precise results in particular in the recognition of thin or transparent adhesive tape, since the reflection and transmission properties in the area of the tape usually do not differ distinctly enough from the other areas of the note without tape.
In other methods, notes under examination are exposed to ultrasound and the sound fraction reflected or transmitted by the note used to calculate the thickness of the note. These methods are also not fully suitable for recognizing adhesive tape on notes since the frequently used commercial adhesive films have very small thickness compared to the paper thickness of the note. The differences of the reflected or transmitted sound in areas with and without adhesive tape are thus accordingly small, so that in particular thin adhesive tape cannot be recognized with sufficient reliability.
It is the problem of the invention to state a method and corresponding apparatus for more reliable recognition of foreign material, in particular undesirable adhesive objects, glue or joints, on bank notes.